This invention relates to a semiconductor laser device consisting of a plurality of independent semiconductor laser chips emitting laser beams, which are close to each other, a method for fabricating same and an optical head using the same.
At the present time, research for the purpose of increasing the speed and improving characteristics of optical disk devices and laser beam purities is actively advancing. Concerning the optical disk devices, several methods for realizing an error check function just after the recording using a plurality of light spots, a recording/reproducing/erasing function or a parallel processing function capable of recording/reproducing simultaneously 2 tracks are known (cf. e.g. JP-A-61-196446). Further, for a laser beam printer, it is intended to increase the recording speed similarly by using a plurality of light spots. As a measure for realizing the methods described above, utilization of a semiconductor laser array has been studied. There are two types of prior art semiconductor laser devices in which the semiconductor laser arrays stated above are mounted, monolithic type and hybrid type, as described e.g. in the preliminary reports of the 47th Meeting of the Applied Physical Society of Japan, 27 p-T-10, 11, p. 159 (1986). In a monolithic type device a plurality of laser elements are arranged in the form of an array on a semiconductor laser chip, for which the lower limit of the interval between adjacent light emitting spots is usually about 100 .mu.m.
In a hybrid type device a plurality of semiconductor laser chips are arranged in parallel on a mounting plane, for which the lower limit of the interval between adjacent light emitting spots is usually about 150 .mu.m.
Further, in the prior art semiconductor laser devices of both types, the oscillation wavelength of a plurality of semiconductor laser elements or chips included in the devices was uniform.
In optical disk devices and laser beam printers it is required that the interval between collected light spots of a plurality of focussed laser light beams be several .mu.m to several tens of .mu.m. Therefore it is desirable that the interval of light emitting spots in the semiconductor laser device be smaller than 100 .mu.m, although it depends on the image magnification of the focussing optical system used in the optical disk device or the laser beam printer. This is because the effective visual field of the optical lens system relating to the semiconductor laser device stated above is limited and it is necessary to use the lens system in its paraxial region where there are no aberrations. This requirement becomes more severe, when an attempt is made to make the optical lens system smaller.
For the prior art monolithic type semiconductor laser arrays, when the interval between light emitting spots, i.e. the array interval of semiconductor laser elements is reduced to a value below 100 .mu.m, and the elements interfere thermally and electrically with each other, which makes it difficult to modulate their output independently. Consequently, in order to reduce the interval between light emitting spots to a value below 100 .mu.m, a problem, which is serious from the technical point of view, needed to be overcome.
On the other hand, for the prior art hybrid type semiconductor laser arrays, where a plurality of laser devices are arranged in parallel on a plane, since it is necessary to cut laser chips at the extreme proximity of the laser active region, deteriorations due to distortions in the laser chips are produced which gives rise to a problem that the laser life is shortened. Consequently it is impossible to reduce the interval between light emitting spots to a value below 100 .mu.m.
However, in order to effect recording and reproduction in real time in an optical disk device without waiting time for rotation of the disk, at least 2 semiconductor lasers are necessary, one having a high output for writing and erasing and another having low noise characteristics for reproduction. Further, when the semiconductor lasers have the same wavelength, it is necessary to separate spatially the beams in the signal detecting system, which complicates the optical system. This is disadvantageous from the point of view of reducing the size and lowering the cost. On the other hand, for the monolithic type it is extremely difficult from the point of view of the semiconductor production process to arrange laser elements having different wavelengths, outputs and noise characteristics in the form of an array.
The interval l between adjacent spots in an optical disk device using a semiconductor laser device as an optical head is determined by the distance d between laser diodes, the numerical aperture NA.sub.c of the collimating lens and the numerical aperture NA.sub.f of the focusing lens as follows: ##EQU1## In order to collect efficiently the beam emitted by the semiconductor laser device so as to achieve recording or erasing, usually a collimating lens having an NA.sub.c, which is greater than e.g. 0.2 is used. For the optical head for recording, a focusing lens having an NA.sub.f which is e.g. 0.5 is frequently used. Therefore, for an optical head using a monolithic type semiconductor laser array and described as an example of the prior art techniques the interval l between spots is greater than 40 .mu.m, when d.gtoreq.100 .mu.m, NA.sub.c .gtoreq.0.2 and NA.sub.f =0.5. When the interval between spots on a surface of the disk is 40 .mu.m, deviation of the track in the optical disk device is about 0.13 .mu.m, which exceeds the tolerable track deviation.